A great deal of effort has been expended to obtain a workable, commercially acceptable process for electroplating nickel-chromium alloys. Various combinations of complexing agents have been used with both trivalent and hexavalent chromium ions. However, it has not been possible until now to produce thick, high quality nickel-chromium deposits with high chromium contents.
The behavior of complex bimetallic electroplating baths is highly unpredictable. Minor changes in electrolyte compositions may result in the formation of different and/or new families of complex ions and may result in significantly poorer coating properties or even a process which will not work at all. Typical of the problems encountered by earlier workers in this field were (a) the inability to build up a coating of thickness greater than 25 microns, (b) inability to incorporate significant amounts of chromium into the deposit and (c) poor morphology, i.e., coatings which are highly stressed or dendritic. More recent efforts to overcome these problems have been partially successful, and have made use of, e.g., iron-nickel-chromium processes, pulsed electrodeposition and/or the use of hexavalent chromium. However, thick coatings of nickel-chromium alloy are very difficult to obtain from hexavalent electrolytes under normal conditions, and the use of hexavalent ions requires more energy for electrodeposition. Moreover, trivalent ions are also far easier to treat when present in electroplating waste than are hexavalent ions.
Although pulsed electrodeposition has been known for quite a long time, the affects on the deposit morphology of varying the parameters involved can only be qualitatively guessed at. Advances in the theory of pulsed electrodeposition still do not permit prediction in advance of either morphology or composition for complex alloy systems such as complex binary chromium alloy baths. Rather, the effects of both composition and variation of electrodeposition parameters for complex multicomponent systems can only be determined by exhaustive experimental work.
It was hoped that electrodeposited coatings of nickel-chromium alloys would have a number of properties which would be likely to attract wide commercial interest if an appropriate deposition process were found. Corrosion resistance was likely to be unusually high for materials with chromium alloy surfaces having a chromium content greater than about 20 weight percent. The wear performance, especially at higher temperatures, might be comparable to other commonly used coatings such as hard chromium and, if the coating could be applied using electrodeposition techniques, the adhesion between the coating and the substrate should be comparable to the cohesive strength of the substrate itself, with the additional advantage that the coating could be applied to the interior surfaces of very small parts. Protective wear- and corrosion- resistant coatings are becoming increasingly important since they permit the production of parts with high corrosion resistance and wear resistance without the necessity of making the entire part out of stainless steel.
A need therefore continues to exist for an efficient, commercially viable process for electrodeposition of thick nickel-chromium alloys having high and controllable chromium contents and excellent surface morphology, corrosion resistance and wear resistance.